This study sought to characterize the impact of tropical system precipitation on streamflow values measured in nine Illinois watersheds and determine whether a predictive model based on antecedent and expected rainfall conditions could be developed. Unlike smaller-scale thunderstorm complexes which are difficult to forecast beyond 24 hours, there is much greater forecast lead time associated with tropical systems. The differences in spatial and temporal scales associated with two types of heavy rain producing storms suggests that hydro-meteorologists have a better opportunity to develop timely regional precipitation and streamflow forecasts as a tropical system approaches Illinois. During a 100-year period (1913-2012) 26 tropical systems were found to have produced an average of an inch or more precipitation within a 24-hour window (i.e., event) in one or more of Illinois’ nine climate divisions. Those climate divisions impacted by an event experienced a significant increase in monthly soil moisture levels as measured by the Palmer Drought Severity Index (PDSI). When pre- versus post-tropical system streamflow (ST) values were compared for the non-event watersheds an increase in ST of less than 50% occurred, while ST changes in watersheds that experienced an event increased by more than 500%. Factors that influenced the magnitude of increase included pre-storm ST conditions (i.e., was ST below, near or above average prior to the event), timing of event (i.e., summer or fall), and total storm precipitation. The predictive ST model, where model output (e.g., post event ST estimates) could be integrated into decision support tools used by those impacted by flooding, was of limited use. Reasons for a lack of success were related to three primary issues, a small sample of events (i.e., 26) in the study, events occurring over a long period (100-years) when changes in land use and agricultural practices altered the surface hydrology (i.e., use of tiles, no-till agriculture, expanding crops in risky environments, etc.), and different surface characteristics (e.g., basin shape and size, soils, geomorphology, etc.) among the nine watersheds. Despite the issues that add complexity to the rainfall to ST relationships over time, those who are tasked with forecasting tropical storm precipitation and related ST values have greater knowledge of how ST values increase and can provide more lead time to regional decision makers in affected watersheds.